This invention relates to friction compositions, particularly asbestos free friction compositions especially suitable for use as disc brake pads or drum shoe lining in the automotive industry.
Because of its assumed chemical and biological inertness, its temperature resistance, its fibrous nature, and its possession of approximately the appropriate degree of hardness and coefficient of friction with ferrous metals, asbestos has been used as a mainstay of friction materials in industry in general and in particular, in the automotive industry. The composition of these materials frequently included additives of various types to improve their bonding characteristics, their noise generating potential, their heat transfer ability, or even to reduce their costs. Among the additives listed in the prior art are zinc oxide and calcium carbonate used in percentages of less than 20% in combination with asbestos or even on occasion with other fibrous materials.
Applicant considers the following U.S. patents to be relevant to his invention or to particularly preferred embodiments thereof. Copies thereof accompany this application.
U.S. Pat. No. 2,078,617--Spokes disclosing the use of cashew nut oil in friction surfaces such as brakes and clutches. The use of asbestos is contemplated (col. 3, line 59).
2,158,337--Rasmussen describes a brake lining having a glass fiber incorporated therein. The glass fiber may be used in place of asbestos and is contemplated as the primary friction material.
2,165,140--Harvey discloses friction elements containing hexamethylene tetramine modified cashew nut oil, and phenolic resins used with fillers in the usual proportions, such as zinc oxide, carbon, barytes and rubber. Fibers other than asbestos may be used.
3,526,306--Bentz et al., discloses a clutch facing which contains glass filaments in combination with asbestos, phenolic resins, butadiene-acrylonitrile rubber, barytes and particulate cashew nut oil.
3,850,874--Grazen et al., discloses a friction element containing a substitute material for the cashew nut oil particles of conventional friction compositions.
3,857,811--Grazen et al., claims the composition of the substitute material used in the 3,850,874 patent.
3,297,599--Eschen discloses the use of calcium aluminate in otherwise conventional brake compositions.
3,916,062--Dougherty discloses the use of molecular sieves in otherwise conventional friction materials, and
4,002,592--Baskin discloses the use of spinels in low concentration as friction modifiers in otherwise conventional friction compositions.
The compositions of this invention employ zinc oxide and/or calcium carbonate and materials with similar properties, as set forth more fully hereinafter, in quantities greater than 20% to totally replace asbestos in friction compositions.
Replacement of asbestos has, of course, become desirable from the standpoint of friction material manufacturers and of those who directly handle the finished product in putting it into the vehicle in which it is used because of respiratory hazards found to be associated with its use and the expense of health precautions now required for its safe handling.